Such a seat is generally known. The peripheral edge of the sitting part in such a seat is generally symmetrical, for example rectangular or U-shaped, and the recess is provided in the centre thereof. GB-A-342.428 discloses a seat having a rectangular peripheral edge and a substantially flat top face comprising two recessed portions. The depressions enable a user of the seat to avoid discomfort arising from pressure upon the pelvis bones. For a good sitting posture, it is important that the pelvis of a person seated on the seat tilts towards a certain position. For the tilting of the pelvis has an effect on the curvatures of the spine of the user. A wrong posture can therefore lead to many back complaints. The tilting of the pelvis is affected to a large extent by the degree of support of the thighs combined with the position of the recess.
A disadvantage in the case of this known seat is that it is intended for sitting on in one position which is determined by the designer. This means that in the case of any person sitting on it only one specific degree of support of the thighs combined with the position of the recess is possible. This support will hardly ever be the most ideal, because no two persons are the same. Each person in fact has a build, posture, weight and height which is characteristic of him/her. For example, the length ratio between thighs and lower legs and between lumbar region of the spine, thoracic region of the spine and cervical region of the spine, and the distance between the seat bones, is different in every case. In the case of each person there should therefore be a characteristic support of the thighs combined with a suitable support of the seat bones, i.e. the buttocks, in the recess.